This invention relates to a hydraulic tensioner having plastic cap mounted at the top of the piston to serve as a vent for oil and air from the piston. More particularly, this invention relates to a hydraulic tensioner in which a plastic cap is fitted over the nose of the piston and permitted to move small distances with respect to the piston in the axial direction. Oil and air that travel through a tortuous path vent at the top of the piston accumulate inside the cap. At a predetermined pressure, the cap moves upward and the accumulated oil and air are permitted to escape.
The plastic cap vent of the present invention operates on tensioners in which the piston is pointed upward when installed in the engine system as well as on tensioners in which the piston is pointed downward when installed in the engine system. When the engine is not operating, the piston spring will force the piston outward against the tensioner arm, which will provide sufficient force to close the plastic cap against the tip of the piston and prevent escape of oil from the high pressure fluid chamber.
A tensioning device, such as a hydraulic tensioner, is used as a control device for a power transmission chain as a chain travels between a plurality of sprockets. As a chain transmits power from a driving sprocket to a driven sprocket, one portion or strand of the chain between the sprockets will be tight while the other portion of the chain will be slack. In order to impart and maintain a certain degree of tension in the slack portion of the chain, a hydraulic tensioner provides a piston that presses against a tensioner arm or other chain guiding mechanism.
Prevention of excess slack in the chain is particularly important in the case of a chain driven camshaft in an internal combustion engine in that a chain without sufficient tension can skip a tooth or otherwise throw off the camshaft timing, possibly causing damage or rendering the engine inoperative. However, in the harsh environment of an internal combustion engine, various factors can cause fluctuations in the chain tension.
For instance, wide variations in temperature and thermal expansion coefficients among the various parts of the engine can cause the chain tension to vary between excessively high or low levels. During prolonged use, wear to the components of the power transmission system can cause a decrease in chain tension. In addition, camshaft and crankshaft induced torsional vibrations cause considerable variations in chain tension. Reverse rotation of an engine, occurring for example in stopping or in failed attempts at starting, can also cause fluctuations in chain tension. For these reasons, a mechanism such as a hydraulic tensioner is desired to ensure the necessary tension on the slack side of the chain.
Hydraulic tensioners are a common method of maintaining proper chain tension. In general, these devices employ a tensioner arm or lever arm that pushes against the chain on the slack side of the chain. This lever arm must push toward the chain, tightening the chain when the chain is slack, and must provide resistive force when the chain tightens.
Typically, a hydraulic tensioner includes a piston in the form of a hollow cylinder. The piston slides within a bore in the housing and is biased outward from the housing in the direction of the tensioner arm and chain by a piston spring. The interior of the piston forms a high pressure fluid chamber with the bore or opening in the housing. The high pressure chamber is connected through a one way check valve to a low pressure chamber or reservoir, which provides or is connected to an exterior source of hydraulic fluid.
Upon start-up, the force of the spring on the piston causes the piston to move further outward as the chain begins to move. Outward movement of the piston creates a low pressure condition in the high pressure fluid chamber, or pressure differential across the inlet check valve. Accordingly, the inlet check valve opens and permits the flow of fluid from the reservoir, or low pressure chamber, into the high pressure chamber. When the high pressure chamber is sufficiently filled with fluid, the force on the chain that moves the piston inward will be balanced by the outward force from the spring and the resistance force of the fluid in the chamber. The force of the chain against the fluid in the chamber also causes the check valve to close, which prevents further addition of fluid to the chamber.
Various types of hydraulic tensioners are described in Suzuki et al., U.S. Pat. No. 5,352,159, Goppett et al., U.S. Pat. No. 4,792,322, and Sosson U.S. Pat. No. 4,850,941. The hydraulic tensioner of Sosson U.S. Pat. No. 4,850,941, has a check valve mounted in the piston, providing a relatively small high pressure chamber. The high pressure chamber is defined by part of the cavity formed in the housing and the piston. The tensioner does not have a spring between the body and the piston or a means for permitting discharge of air from the chamber.
U.S. Pat. No. 4,826,470 discloses a hydraulic tensioner with a check valve mounted in the nose of a piston. The check valve permits air to escape from the piston. U.S. Pat. No. 4,507,103 discloses a hydraulic tensioner with a check valve and vent in series. The check valve has a low opening pressure so that fluid flows from the high pressure chamber through the check valve and then to the tortuous vent path to atmosphere.